Phased array technology has been in existence for decades and provides an electronic means for aperture synthesis by virtue of electronic control of array element amplitudes and relative phases. As is well known in the prior art, the steady state far field beam pattern of a discrete array of equidistant emitting elements comprising a phased array, is obtained by the Fourier transform of the complex aperture weights (discrete apodization function) of the array. Hence, the desired beam patterns can be synthesized for both transmission and reception based on the application of appropriate amplifier gains and phase shifter values to each respective element of the array. Further, M−1 beams can be independently and concurrently synthesized with the degrees-of-freedom provided by an array of M elements. Time varying adjustment of the complex weights of the array allows time variation of the patterns and more specifically, provides for multiple target tracking. Phased arrays have been implemented to provide agile beam control for radar, sonar, and lidar. Two-dimensional phased arrays serve as the basis for many surveillance and tactical radar systems requiring increased resolution and beamsteering agility.
A number of issues attend the implementation of phased arrays. Foremost among these is that the optimum lateral spacing of array elements is one-half wavelength of the radiation to be emitted. This mitigates the appearance of grating lobes in the beam pattern. Because the array resolution (achievable narrowness of beamwidth) along a single lateral axis of the array is proportional to the number of array elements along that axis for a fixed interelement spacing, the single axis dimension of the array is governed by both wavelength and desired resolution of the array. At UHF and VHF frequencies, the need for high resolution implies a very large physical footprint for the array. A secondary issue for arrays is the process of beamforming over substantial bandwidths. In the wideband case, due to dispersion, the array weights become frequency-dependent functions and to treat them as constants would lead to the occurrence of severe beam distortion over frequency. Hence, the array weights are implemented as wideband filter functions with the attending complication. Finally, there is the issue of the behavior of the individual array elements. In the electromagnetic array implementation, wherein the array elements comprise antenna elements, there can be mutual coupling between these elements that can lead to beam pattern distortion. This requires introduction of means to achieve decoupling. In the case where such coupling is linear with power and frequency, this can be achieved notionally by processing the signal array with a decoupling matrix. Likewise, if the coupling is frequency dependent, then so is the necessary decoupling process and the decoupling matrix is a frequency-dependent matrix filter function.
It would be extremely useful to implement methods of phased array system design that can overcome the size and wideband processing constraints of the prior art as applied to radar, sonar, and communications systems. To do so, would greatly increase the achievable resolution of such arrays of a given size. Further, means to obviate the necessity for wideband signal processing would significantly simplify analog and rf hardware designs for array systems as well as alleviate the associated signal processing burden. The combination of such methods for receive arrays along with transmit antenna advancements can make feasible the application of phased arrays to mobile communications platforms.
As will be discussed below, a central aspect of the present invention is the exploitation of the received energy geometry phase. One instance known to the authors of the association of differential phase among elements of an array with the energy received from a target is made on page 12 of the dissertation by Jeffrey T. Carlo, entitled “Direct Data Domain Approach Using Nonlinear Arrays,” Syracuse University, August 2003. In contrast to the present invention this association enabled removal of target signal energy from the data to be processed.